1. Field of the Invention
The present invention relates to an optical dropping apparatus for dropping optical signals from wavelength-division multiplexing signals by using four-wave mixing and further to an optical add/drop multiplexer for dropping/adding/passing wavelength-division multiplexing signals by using four-wave mixing.
Ultra-long-distance and large-capacity optical communication apparatuses are now required to construct future multimedia networks. Concentrated studies are now being made of the wavelength-division multiplexing as a method for realizing large-capacity apparatuses in view of such advantages that it can effectively utilize a wide bandwidth and a large capacity of an optical fiber.
Especially in recent years, there has been demanded not only the optical communication system for sending/receiving a WDM optical signal between two terminal stations but also the optical communication system having the ADM (Add-Drop Multiplexer) function through the repeater stations called the xe2x80x9cnodesxe2x80x9d and disposed midway of an optical transmission line. This ADM function is to selectively pass only an optical signal having a special wavelength, of the wavelengh-division multiplexing optical signals, to drop the optical signals of the remaining wavelengths with the nodes, and to add another optical signal from that node and send it to another node. For these functions, there have been vigorously investigated the optical add/drop multiplexer (as will be abbreviated as the xe2x80x9cOADMxe2x80x9d) having the ADM functions and acting as the key device of the optical communication system.
2. Description of the Related Art
This OADM is disclosed, for example, in Laid-open Japanese Patent Application Publication No. 11-055184.
In an OADM 120, as shown in FIG. 13, a WDM optical signal from the precedent node are inputted into one T1 of three ports T1, T2 and T3 of an optical circulator (as will be abbreviated as the xe2x80x9cOCxe2x80x9d) 110.
In this OC 110: the optical beam inputted from the port T1 is outputted to the port T2; the optical beam inputted from the port T2 is outputted to the port T3; and the optical beam inputted from the port T3 is outputted to the port T1. The port T2 of the OC 110 is connected with an optical fiber Bragg grating filter 111 (as will be abbreviated as the xe2x80x9cFBGxe2x80x9d) for reflecting only an optical beam of a predetermined wavelength xcexy and for passing optical beams of the remaining wavelengths. The port T3 of the OC 110 is connected with optical receiving circuit for receiving/processing the optical signals.
The optical signals of the remaining wavelengths (except the optical signal of the wavelength xcexy) passed through the FBG 111 are inputted into an FBG 113 through an optical isolator (as will be abbreviated as the xe2x80x9cISOxe2x80x9d) 112 for passing an optical beam only in one direction. The FBG 113 also reflects only the optical beam of the predetermined wavelength xcexy and passes the optical beams of the remaining wavelengths.
The optical signals passed through the FBG 113 are multiplexed with the optical signal of the wavelength xcexy inputted from the port T3 of an OC 114 having actions similar to those of the OC 110. The optical signal thus multiplexed is inputted into the port T1 of the OC 114 and is outputted from the port T2 to a subsequent node. On the other hand, the port T3 of the OC 114 is connected with an optical sending circuit for sending the optical signal of the wavelength xcexy. Here, the ISO 112 prevents the multiple reflection between the FBG 111 and the FBG 113.
In this OADM 120, the optical signal having the predetermined wavelength xcexy of the inputted WDM optical signal is reflected on the FBG 111 and inputted into the port T2 of the OC 110. Moreover, the optical signal of the wavelength xcexy is dropped by the OC 110 from the port T2 to the port T3. On the other hand, the optical signals of the remaining wavelengths passed through the FBG 111 are multiplexed with the optical signal of the wavelength xcexy, as added from the port T3 of the OC 114, and are outputted from the port T2 of the OC 114 to another node.
Here in the OADM 120 shown in FIG. 13, the reflection wavelengths of the FBGS 111 and 113 are fixed. As a result, the wavelength xcexy of the optical signal dropped/added is determined at the time of constructing an optical transmission system so that the wavelength of the optical signal dropped/added cannot be arbitrarily changed during the time the optical transmission system is running.
When a plurality of wavelengths are to be dropped/added, moreover, the number of OADMs 120 required is equal to that of the optical signals to be dropped/added, so that the optical circuit is required to have more parts and complicated more.
An object of the invention is to provide an optical dropping apparatus which is able to drop an optical signal of an arbitrary wavelength from a WDM optical signal by using four-wave mixing in optical fibers.
Another object of the invention is to provide an optical adding/dropping apparatus which is able to add/drop an optical signal of an arbitrary wavelength from WDM optical signal by using the four-wave mixing in optical fibers.
Another object of the invention is to provide an optical dropping apparatus which is able to drop a plurality of optical signals having arbitrary wavelengths by using four-wave mixing in optical fibers.
And another object of the invention is to provide an optical adding/dropping apparatus which is able to add/drop a plurality of optical signals having arbitrary wavelengths by using the four-wave mixing in optical fibers.
The above-specified objects can be achieved by an apparatus comprising: generating part for four-wave mixing; dropping part for dropping light of a predetermined wavelength; and controlling part for controlling the wavelength of pump light which is used for four-wave mixing in the generating part.
In this apparatus, light of the predetermined wavelength is dropped from the lights generated by four-wave mixing. In this apparatus, moreover, the wavelength of the optical signals, before occurring four-wave mixing between pump light, can be changed by changing the wavelength of the pump light, so light of an arbitrary wavelength can be dropped.
In this apparatus, on the other hand, the generating part four-wave mixing occurs only in the light of the predetermined wavelength, of a plurality lights, when one pump light acts on the lights of a plurality of wavelengths.
This apparatus can drop/add a light from the light to occur four-wave mixing. Alternatively, the apparatus can drop the light generated by four-wave mixing and can drop/add one of the lights in which four-wave mixing occured. As a result, the apparatus can drop/add a plurality of lights of arbitrary wavelengths.